This invention relates to antennas and devices incorporating antennas. In particular, this invention relates to low cost passive, true time delay beamformers that can be used to feed an antenna array.
Like other electronic components and systems, the speed, complexity, and component density in microwave and millimeter-wave systems have been ever-increasing. With the increasing number and variety of components, controllers, and connections, the power consumption and noise and other interference problems of these systems have correspondingly increased. One and two dimensional electronically scanned arrays, i.e. beamformers, are integral components of these systems. The beamformer uses a limited number of control signals to control multiple time delay components (phase shifters) distributed into a fractal RF feed network and thereby scan the main beam of the beamformer.
Conventional phase shifters use relatively bulky, expensive perturbers that are external to the actual phase shifters (the substrate containing the feed network or the antenna array) to modify the electrical characteristics of transmission lines in the phase shifters. Needless to say, conventional phase shifters are in general difficult and expensive to fabricate. Conventional phase shifters are also generally RF-active devices that require a comparatively large amount of power and may interfere with the transmitted signal. In addition, because conventional phase shifters alter the phase of an input signal thereby only simulating a time delay, a fixed, progressive time delay between elements is obtained only over a relatively narrow band of frequencies. As a consequence, if the frequency of the beam wanders, the pointing angle wanders correspondingly.
Thus, a beamformer that employs conventional phase shifters only forms a beam at essentially one frequency or a narrow band of frequencies; if the frequency transmitted changes substantially, the antenna element spacing must be either physically moved or the phases set by the phase controllers changed to form a beam at the new frequency (in a controllable-type beamformer array). This process may be time consuming and awkward or even physically impossible. Further, this is increasingly important for systems communicating at frequencies that are relatively far apart. Some existing and proposed earth-orbiting satellite communication systems communicate simultaneously at approximately 20 and 30 GHz.
Accordingly, variable true time delay devices, as well as beamformers that employ the variable true time delay devices, are desirable: they have low power consumption, decreased interference, are low-cost, and have a given pointing angle over a broad band of frequencies.
To provide these and other objects presented herein, the variable true time delay device comprises a fixed medium having a first conductive path along which electromagnetic signals propagate, a movable medium having a second conductive path along which the signals propagate, and, in some cases, a thin dielectric layer disposed between the fixed and movable media. The movable medium is translatable such that the second conductive path overlaps the first conductive path by a variable amount. The time delay through the device is dependent on the overlap between the first and second conductive paths.
The first and second conductive paths may be printed traces such as used in microstrip, stripline, or coplanar waveguide transmission lines. The movable medium may be linearly translatable by an actuator. Either or both of the first and second conductive paths may comprise a U-shaped path which we denote as a trombone line.
The first conductive path may comprise four sections of different widths in which pairs of the sections symmetric around a center line have the same length. Similarly, the second conductive path may comprise sections having the same length, symmetric around the center line, and overlapping one pair of the four sections. The lengths and widths of the sections of the first and second conductive paths may be selected to implement an impedance match between ends of the first conductive paths.
In some embodiments, no direct or ohmic contact is required between the first (fixed) and second (movable) conductive paths. The movable medium may have dielectric materials whose permittivity is much lower than that of the fixed medium, and comprise a sliding stop to prevent overrun of the first conductive path by the second conductive path.
Beamformers may use any of the above phase shifters. The beamformer may, for small scan angles, have a scan angle defined by:   θ  =      arcsin    ⁡          (                                    4            ⁢            Δ                    d                ⁢                              ϵ            eff                              )      
where xcex94 is the physical displacement of the second conductive path, d is an inter-element spacing between antenna elements of the beamformer, and xcex5eff is an effective dielectric constant of a feed network of the beamformer.
The beamformer may require only one actuator per dimension of beam forming.